Various processes for the toning of tacky, particularly imagewise tacky, surfaces are known from U.S. Pat. Nos. 3,060,024; 3,391,455; 3,506,483; 3,637,385; 3,649,268 and 4,019,821. In these known processes, the toning is performed with the use of mechanical devices to distribute and incorporate the toner particles in the tacky surface areas by means of brushes, cotton swabs, etc. A prerequisite for the practicability of this process is a distinct difference in hardness between the tacky and nontacky surface areas. Due to the hardness of the nontacky surface areas, there is no risk that the surface in these nontacky areas is damaged by mechanical forces during toning and that toner particles are incorporated in an undesirable manner. If the nontacky surface areas have sufficient hardness, toner particles, which remain on the nontacky surface after toning and thus interfere with the powder image, can be removed with the use of mechanical means, for example, wiped off.
However, these cited processes are disadvantageous, due to the mechanical energy used if extremely finely divided toner (about 1 to 2 .mu.m average grain size) has to be applied, if the hardness in the nontacky surface area is low, if uneven, curved or dome-shaped surfaces have to be toned or particularly, if there is little difference in hardness between the tacky and nontacky surface areas. The size of the bristles or the fibers in the brushes or swabs is at least several micrometers (.mu.m), so that the so-called stain, a smear from the finest particles, cannot be wiped away by these means after toning. Moreover, these materials are so hard that the nontacky but relatively soft surface can be damaged easily and the fine toner particles can collect in these damage indentations. Furthermore, brushes and swabs adapt poorly or not at all to textured or uneven surfaces.
Other known toning processes are based on the generation and use of a free-flowing toner layer, for example, immersion of the surface to be toned in a fluidized bed (R. W. Peiffer and R. W. Woodruff, Research Disclosure 15882, June, 1977); these result in powder images of lower coverage. Free-flowing toner zones are useful rather for the toning of surfaces differing little in hardness, but much in tackiness. However, the disadvantage of these methods is that, due to the highly dispersed toner, particulate images of extremely low volume density and low uniformity result.
Finally, U.S. Pat. No. 4,687,825 discloses a process for coating fluorescent screens with phosphor, in which process the fluorescent screen to be coated, on an axis right-angled on all sides to its previously tackified surface to be coated, is rotated at an incline to this axis and the phosphor is applied onto this surface as fine particles before or during rotation. To facilitate the distribution of the phosphor over the surface to be coated, the fluorescent screen can also be vibrated during rotation. In this process as well, only very low volume density and low coating uniformity can be attained, because, during the rotation of the fluorescent screen, the coating material, that is, the phosphor, only slides off the tacky surface; this sliding effect is even increased by the vibration, and the tendency of the phosphor to adhere to the tacky surface is reduced.
The low volume density attainable with fluidized bed coating and rotation coating is not suitable for many applications. This applies particularly, for example, when metal powder patterns have to be converted by fusion into continuous metal films.
Consequently, an object of the invention is to provide a process for the coating of tacky or previously tackified surfaces, with which process imagewise coatings are prepared from finely divided powders in the tacky areas with a uniform powder image of high packing density and high toner quantity takeup with good reproducibility, even if the tacky areas and nontacky areas differ only slightly in hardness, in which process no adhering particles or only particles adhering by electrostatic charge remain in the nontacky areas and all constituents of the toner are incorporated in the same manner in the tacky areas so that no change in the toner is supposed to occur in its composition and its properties.